CANONSBURG, Pa. (AP) -- The Republican Party will watch the final vote-counting in a tightly contested U.S. House race in Pennsylvania before deciding whether to seek a recount or sue over perceived election irregularities, officials said Thursday, even as they scrounged for votes to whittle away at Democrat Conor Lamb's lead.

But changing a final count by hundreds of votes, such as Lamb's lead over Republican Rick Saccone, is unheard of in Pennsylvania on electronic voting machines like ones used in Tuesday's election, county officials and election law specialists say.

With absentee ballots counted, Lamb, a 33-year-old former prosecutor and first-time candidate, held a lead over Saccone of 627 votes out of more than 228,000 cast, according to unofficial results.

Election officials in the four counties in the Pittsburgh-area district had identified about 400 uncounted provisional, military and overseas ballots by Thursday.

The Associated Press has not called the race. But Lamb's lead in a long-held Republican district that strongly backed President Donald Trump in 2016 has sent waves of fear through Republicans nationally as they try to hold on to their 45-seat U.S. House majority in November's mid-terms.

Lamb has declared victory. Saccone, a 60-year-old Air Force veteran turned state lawmaker and college instructor, hasn't conceded. Saccone's campaign said he had no plans to concede before vote counting finishes.

Vote counting should wrap up next Tuesday, before Republicans decide whether to sue or seek a recount.

"Every vote needs to be counted and that process needs to unfold," said Michael Stoll, the executive director of the state Republican Party.

In the meantime, the National Republican Campaign Committee launched Facebook ads Thursday in an effort to reach 200,000 Republican voters in the district to see if they had problems at their polling location. It is designed to connect them to a committee lawyer.

Still, county officials and election lawyers in Pennsylvania said Thursday that they could not think of a time when a recount involving electronic voting machines changed the result of an election by more than a few votes.

Larry Otter, a lawyer who specializes in Pennsylvania election law and recounts, said the only thing he could imagine that might substantially change a vote count is if a poll worker misread the paper tape from the electronic voting machine.

That, however, would presumably be caught when the counties, under state law, perform an audit of the results on the electronic voting machines, beginning Friday. That typically involves comparing the overall tally on a hard drive, a flash drive and a paper tape that separately record each vote.

Gregory Harvey, another lawyer who specializes in Pennsylvania election law and recounts, said the gospel among election lawyers is that recount drives are intended to raise campaign donations.

"It's all a fundraising scam," Harvey said.

Some Republican officials in Pennsylvania privately expressed skepticism about overturning a Lamb victory, as Saccone himself focused on signature-gathering to get on the May 15 primary ballot to run for a full term.